Continental Reactivation and Reworking: an Introduction

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Continental reactivation and reworking: an introduction

R. E. HOLDSWORTH l, M. HAND 2, J. A. MILLER 3 & I. S. BUICK 4

1 Reactivation Research Group, Department of Geological Sciences, Durham University, Durham DH1 3LE, UK 2 Department of Geology and Geophysics, Adelaide University, Adelaide, SA 5005, Australia 3 Department of Geological Sciences, The University of Cape Town, Rondebosch 7700, Republic of South Africa 4 Department of Earth Sciences, La Trobe University, Bundoora, VIC 3083, Australia

In contrast to oceanic lithosphere, the continents are linked to the on-going Alpine collision (e.g. are manifestly composed of the products of tec- Ramandi 1998). In the ancient geological record, tonic processes whose cumulative duration spans two of the best examples are the mid-Palaeozoic much of the Earths history. Most continents Alice Springs Orogeny and the Neoproterozoic contain Archaean nuclei that are enclosed by to Palaeozoic Petermann Orogeny in central Proterozoic and Phanerozoic tectonic domains. Australia (e.g. Sandiford & Hand 1998; Hand & The evolution of post-Archaean continental Sandiford 1999). In recognition of the impor- volumes has included additions of new continen- tance of intraplate orogeny as an expression tal material, but it has also involved repeated of continental rejuvenation, a large amount of modification of parts of the existing continental work has focused on the mechanisms leading lithosphere during periods of tectonic rejuvena- to large-scale intraplate failure (e.g. Vilotte et al. tion. This generally involves processes such as the 1982; England & Houseman 1985; Kuzsnir formation of new structural fabrics, the over- & Park 1987; England & Jackson 1989; Platt & printing of metamorphic assemblages and the England 1994; Tommasi et al. 1995; Ziegler et al. generation and emplacement of magmas. Such 1995, 1998; Avouac & Burov 1996; Neil & behaviour can occur repeatedly throughout the Houseman 1997; Sandiford & Hand 1998; geological record because the quartzofeldspathic Hand & Sandiford 1999; Pysklywec et al. 2000). continental crust cannot be subducted due to A number of factors are likely to control the its relative buoyancy and weakness compared locus of tectonic activity, but there appear to be with its oceanic counterpart and the underlying two first order controls: (1) temporal and spatial lithospheric mantle. Thus, the character of the variations in the thermal state of the lithosphere continents is significantly influenced by the way (e.g. Sonder & England 1986; England 1987; in which the existing lithosphere responds to Neil & Houseman 1997); and (2) the presence new tectonothermal events that follow geologi- of pre-existing mechanical defects such as faults, cally significant cessations of activity for mil- shear zones or major compositional boundaries lions to hundreds of millions of years (Sutton & (e.g. Ziegler et al. 1995; Butler et al. 1997; Holds- Watson 1986). worth et al. 1997). Existing continental lithosphere may be mod- The rejuvenation of pre-existing crust and ified during its incorporation into new collisional lithosphere occurs largely via two related pro- systems, for example the involvement of the cesses. Reactivation is normally considered to Hercynian 'basement' in the Alpine collision. involve the rejuvenation of discrete structures However, the most dramatic manifestations of (e.g. Holdsworth et al. 1997), whilst reworking continental tectonic rejuvenation occur during involves the repeated focusing of metamorphism, intraplate orogeny, where a coherent pre-existing deformation and magmatism into the same lithospheric volume undergoes large-scale failure. crustal- or lithospheric-scale volume. These are Notable modern examples of intraplate orogeny considered to be useful end-member definitions are the Cenozoic Tien Shan and the Mongolian that describe the way in which continental litho- Alti in north Asia, which are forming in response sphere is modified. However, there is some ambi- to the Himalayan collision (e.g. Hendrix et al. guity firstly because reworking and reactivation 1992; Dickson Cunningham et al. 1996), and may represent broadly the same process operat- also the Atlas Mountains of Morocco, which ing at different scales and/or depths (see below),

From: MILLER, J. A., HOLDSWORTH,R. E., BUICK,I. S. & HAND, M. (eds) ContinentalReactivation and Reworking. Geological Society, London, Special Publications, 184, 1-12. 1-86239-080-0/01/$15.00 © The Geological Society of London 2001. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

2 R. E. HOLDSWORTH ET AL. and secondly because there is some confusion the distribution and intensity of crustal heat regarding the use of these terms in the geological production is strongly controlled by the pattern literature. Several studies refer to the 'reactiva- and extent of denudation associated with orogen- tion' of mobile belts, but as these represent esis, which in turn may influence subsequent crustal volumes rather than discrete structures, spatial and temporal variations in the litho- they would be more accurately described as spheric thermal regime (e.g. Sandiford & Hand being reworked. Other terms used in the litera- 1998). Thus the consequences of one stage of con- ture include 'renewal' and 'remobilisation', both tinental evolution can potentially shape the pat- of which refer to the superposition of younger tern, style and distribution of subsequent events. geological events onto older geological systems. Since continental reworking involves diffuse The published geological literature suggests lithospheric-scale deformation, the behaviour of that continental reworking and reactivation can the mantle lithosphere is likely to be central in be expressed in a large number of ways, and determining the style and duration of the tec- are likely to arise for a range of complex rea- tonic activity. The potential role of the mantle sons. Thus, attempts to characterize the style lithosphere in continental reworking is reviewed and distribution of reactivation and reworking and investigated by Houseman & Molnar in the in different continental settings should provide first paper of this volume. They suggest that key datasets with which to evaluate the nature, localized lithospheric thickening associated with distribution and dynamic controls of tectonic plate convergence can potentially affect the gravi- rejuvenation in the continental crust and litho- tational stability of a layered system in which sphere. In editing this book, two end-member a dense non-Newtonian lithosphere overlies a approaches are recognized. The first takes a less dense fluid asthenosphere. Where instabil- lithospheric-scale perspective and essentially con- ity arises, a relatively strong crust will lead to siders how the continental lithosphere may localized downwelling beneath the centre of the respond to evolving first order variables, such convergent zone, whilst a relatively weak crust as the density structure of the mantle and changes will lead to downwelling on the margins of the in convergence style and rate. The second convergent zone as the buoyant crustal layer approach is case-study oriented. The two resists thickening. The initial instability may approaches are complementary and wherever then trigger rapid extension of the lithospheric possible should be integrated. Case studies pro- mantle beneath the orogen which is driven by vide crucial temporal and spatial information asymmetric cold downwellings that move away concerning the thermo-mechanical evolution of from the centre of the convergent zone. Using the crust and mantle in a variety of settings. examples of modern orogens from Southern Geochronological studies are particularly impor- California, South Island New Zealand, the tant in this context as they can provide clear Mediterranean, and Central Asia, Houseman & evidence for reworking or reactivation, and an Molnar show that there is strong evidence in each absolute timescale of events. The conceptual case that the mantle lithosphere has devel- approach can and should provide the stimulus oped some form of instability that has led to at for the collection of targeted datasets that seek least part of it being replaced by hot astheno- to evaluate the interactions of potential beha- sphere. Interestingly, teleseismic tomographic viours and causative mechanisms. images from these areas suggest that mantle lithosphere has been locally renewed following Continental reworking gravitational instability triggered by orogenic convergence. In this context, the time scales of Continental reworking encompasses structural, reworking should be linked to the rates at which metamorphic and magmatic processes that mod- downwelling of gravitationally unstable litho- ify existing continental lithosphere at an orogenic sphere occurs. Although these time scales are scale. An important difference between rework- not well known, Pysklywec et al. (2000) have ing and reactivation is that older structures do not suggested that crustal deformation driven by necessarily control the style, orientation and scale this process should have a duration of c. 25- of later structures. This is evidenced by orogenic 40 Ma. Rey also considers the interplay between belts such as the Cambrian Prydz-Leeuwin oro- the convective and lithospheric mantle in terms genic system which linked Africa, Antarctica and of its potential role in controlling the style of Australia, incorporating Archaean and Proter- crustal deformation. In contrast to Houseman ozoic crustal domains into a single mountain belt & Molnar, he investigates the consequences (Veevers 2000). However, there may be important of mantle behaviour for crustal extension, and long-term linkages or feedback between appar- evaluates the boundary conditions that may ently unrelated orogenic episodes. For example, limit, or oppose spontaneous extension. 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CONTINENTAL REACTIVATION AND REWORKING: AN INTRODUCTION 3

Although the dynamic behaviour of the man- also strengthen the lithosphere, whilst proximity tle is likely to be of primary importance dur- to 'strong' Archaean cratons and high mantle ing continental reworking, the evolution of the heat flow appear to markedly enhance suscept- crustal component of the system also potentially ibility to reworking. exerts long-term controls on the stability of Within the context of a single period of orog- continental lithosphere. For example, crustal enic evolution, many systems appear to record heat production in the Australian Proterozoic is pulses of activity during which the rates and roughly twice the global Proterozoic average intensity of deformation, metamorphism and (Taylor & McLennan 1985), implying that litho- magmatism increase. In the context of the present spheric thermal regimes, and by inference mech- volume, these pulses of tectonic activity could be anical strength, are strongly controlled by the thought of as short-term or short-spaced cycles of thermal character of the crust (e.g. Sandiford & reworking if there has been a local cessation Hand 1998). The potential importance of the of tectonic activity. The existence of apparently deep crustal evolution is highlighted by Ryan, discrete events is not surprising given the com- who points out that as collisions thicken the crust plexity of large orogenic systems and the non- by a factor of two or more, a large volume of uniformity of their boundary conditions. More continental material at the base of the orogen will controversial, however, is the suggestion that experience eclogite-facies conditions. This dense orogenic systems may display episodic (i.e. quasi- crustal material may be partially subducted and regular) pulses of behaviour (e.g. Stfiwe et al. lost to the system, or may sit isostatically below 1993). For example, Bell and co-workers have the Moho until partially exhumed during orog- suggested that in a number of orogenic belts, enic collapse. Using finite element models, he foliations appear to have formed during repeated shows that the rate at which eclogite phase episodes of convergence and extension that can transformations take place can have profound be correlated over large distances (e.g. Bell & buffering effects on the amount and duration of Mares 1999). In some instances, these episodes orogenic contraction. Such remnant orogenic appear to have occurred within the time scale of a roots may exist as seismically reflective mantle single metamorphic cycle. Whether this apparent and provide a locus for subsequent rifting. behaviour is truly episodic is yet to be deter- In some respects, continental reworking at mined, but if it is, the periodicity of the cycles convergent margins is a matter of chance that holds importance clues regarding the dynamic is determined by the presence and positions of behaviour of orogenic systems. Lister et aL high- the colliding continental blocks, arcs, etc. How- light the episodicity of deformation and meta- ever, the spatial patterns of intraplate failure morphic mineral growth in orogenic belts and have attracted considerable attention (e.g. Zieg- show that these events appear to take place at the ler et al. 1995; Hand & Sandiford 1999) from the same time over large length scales, even in appar- perspective of trying to determine whether re- ently unrelated segments of the same orogenic working is controlled by exclusively intraplate belt. They relate this periodicity to switches be- processes (e.g. Neil & Houseman 1999) or if it is tween compressional orogenic surges and peri- driven by stresses originating at plate bound- ods of lithospheric extension following accretion. aries that are focused into a weak zone within They propose that the effect of these switches the continental interior. One intriguing aspect of is greatest in back-arc environments where roll- the spatial patterns of continental reworking back of the subducting lithospheric slab ensures relates to the reasons why the Archaean 'shields' a large amount of lithospheric extension after have survived relatively intact, apparently exhi- each contractional accretion. This could explain biting long-term strength. Krabbendam suggests why the majority of well-preserved examples that the susceptibility of continental lithosphere of exhumed high-pressure metamorphic terranes to reworking is largely determined by the geother- and ophiolite sheets found in the geological mal gradient and rheological properties. Rela- record were formed in back-arc settings. tively strong orogenic lithosphere will occur if the The mechanisms that could drive relatively crust has low rates of radiogenic heat production, short-term episodic behaviour within long-lived or if the underlying sub-continental lithosphere orogens may differ importantly from the pro- remains thick. Low heat production rates may cesses that lead to the formation of new orogenic be a consequence of the average level of denuda- systems. In the former situation, the lithosphere tion associated with earlier orogenic events (e.g. is presumably in a non-equilibrium state. In the Sandiford & Hand 1998), and therefore arise as latter case, there may have been no tectonic a long-term consequence of the terrain history. activity for hundreds of millions of years, and Processes such as dehydration metamorphism the lithosphere may have been in a quasi- and erosional thinning of the orogenic crust can equilibrium condition. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

4 R. E. HOLDSWORTH ET AL.

Reactivation case, reactivation can be viewed as being the accommodation of displacement along a struc- In general, reactivation involves the structural ture formed during an earlier tectonic regime modification of an existing feature without sig- (Holdsworth et al. 1997). nificantly changing its volume or orientation. Perhaps the most obvious example is the reactivation of a shear zone or fault system, in which Continental reactivation and reworking: a younger episode of deformation is localized a central Australian perspective within or along the boundaries of the existing structure. Holdsworth et al. review the nature, Central Australia arguably contains one of the distribution and underlying controls of defor- best records of continental reactivation and re- mation processes and products in natural faults working. This record is expressed by the forma- and shear zones. Reactivated faults and shear tion of two spectacular orogenic systems: the late zones exposed in the deeply exhumed parts of Neoproterozoic to early Palaeozoic Petermann ancient orogenic belts present particularly useful Orogen (e.g. Sandiford & Hand 1998) and the opportunities to study processes that influence mid Palaeozoic Alice Springs Orogen (e.g. Shaw the mechanical properties of long-lived fault et al. 1992). These intraplate orogenic events zones at different palaeo-depths. A case study overprint complex Palaeo- and Mesoproterozoic from Scotland is used to emphasize the impor- orogenic belts, and thus central Australia can tant roles played by cataclasis and fluid flow in be viewed as a type example in which a series the deeper part of the frictional crust, processes of superimposed episodes have occurred during that help to promote retrograde metamorphism continental rejuvenation. Several studies have and changes in deformation regime (see also focused on the factors that may have controlled Imber et al. 1997, 2001; Stewart et al. 2000). the shifting patterns of continental reworking in This in turn leads to changes in the depth central Australia and have emphasized the pri- and character of the frictional-viscous (brittle- mary importance of temporal and spatial varia- ductile) transition along such faults, and to tions in the thermal regime (e.g. Sandiford & profound weakening that may account for the Hand 1998; Hand & Sandiford 1999; Fig. 1), long-lived character of many crustal-scale struc- and/or the role of the lithospheric mantle in tures, including the much studied San Andreas driving deformation (Braun & Shaw 1998; Neil fault zone. In addition to the traditionally recog- & Houseman 1999). nized lithological and environmental factors that In the continued effort to understand the influence fault rock rheology (e.g. composition, processes that lead to reworking of the central grain-size, temperature, pressure, etc), geometric Australian lithosphere, Roberts & Houseman factors such as fault size, orientation and inter- focus on the mid-Palaeozoic Alice Springs Orog- connectivity are likely to be of equal impor- eny, which is the last major event to have affected tance in determining the repeated localization the region. They employ a thin viscous sheet of displacements (e.g. Walsh et al. 2001). The approximation of continental lithosphere to longevity of continental lithosphere means that investigate possible causative mechanisms for once major faults form, they are likely to persist N-S shortening during the Alice Springs Orog- unless there is pervasive re-structuring of the eny, and the contemporaneous development of lithosphere. Thus, many continental fault zones N-S extension in the Canning Basin in north- have very long-lived movement histories (e.g. see western Australia. Assuming an orogenic time Butler et al. 1997; Rutter et al. 2001 and asso- span of 100Ma (e.g. Shaw et al. 1992), they ciated papers). impose a clockwise rotation of the northern In ancient settings, reactivation should refer boundary of a lithospheric plate in which there is to deformation events that are separated by an E-W trending internal weak zone correspond- more than 1 Ma (Holdsworth et al. 1997) since ing to the location of the Alice Springs Orogen. geochronological techniques generally cannot They show that this can produce a realistic resolve events that are separated by shorter amount of crustal thickening in the region periods of time. In more recent settings, where representing central Australia and thinning in the timing of events may be more precisely the region representing the Canning Basin. The defined, it is useful to view reactivation as being rotation may be induced either by eastward shear distinct from recurrence. It is well estab- traction or by a clockwise bending moment. lished that many faults exhibit stick-slip beha- The degree of crustal thickening is primarily viour, with short-lived periods of inactivity controlled by the relative dimension of the (c. 0.01 Ma) within what is essentially a contin- intracratonic weak zone, which is itself defined uous slip history (e.g. Rutter et al. 2001). In this by Moho temperatures that are estimated to be Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

CONTINENTAL REACTIVATION AND REWORKING: AN INTRODUCTION 5

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40 0 50 100 150 200 250km Fig. 1. (a) Generalized map of central Australia showing the location of major crustal blocks and Neoproterozoic to mid-Palaeozoic sedimentary basins that belonged to the formerly continuous Centralian Superbasin. Large- scale intraplate deformation during the late Neoproterozoic to Palaeozoic (580-530 Ma) Petermann Orogeny and the 400-300 Ma Alice Springs Orogeny resulted in exhumation of the Musgrave and Arunta Blocks from beneath the Centralian Superbasin. The Petermann Orogeny produced regional exhumation from depths in excess of 45 km, while maximum exhumation arising from the Alice Springs Orogeny is in the order of 25 km. (b) and (e) Generalized isopach maps within the Amadeus Basin prior to the onset of the Petermann Orogeny (b) and the Alice Springs Orogeny (c) (from Sandiford & Hand 1998). The isopach data indicate that prior to the Petermann Orogeny, the Musgrave Block was buried beneath a relatively thick sedimentary blanket that accumulated over an interval of c. 200 Ma. Following the Petermann Orogeny, the locus of sedimentation shifted such that the Arunta Block became the location of a long-lived depocentre where in excess of 8 km of sediment accumulated. The line of section used to model the thermal response to variations in sedimentary thickness shown in Figures ld and le is indicated by X-Y. (d) and (e) Simplified 2D thermal structure across the Amadeus Basin along the line X-Y (d) prior to the Petermann Orogeny and (e) the Alice Springs Orogeny. The grey region in (d) indicates the location of the Petermann Orogen along the southern margin of the Amadeus Basin. Similarly, the grey region in (e) shows the location of the Alice Springs Orogeny. Both the Petermann and Alice Springs Orogeny appear to have been localized in regions with relatively elevated Moho temperatures, suggesting that the change in locus of intracratonic deformation between c. 530 Ma and 400 Ma may have reflected spatial and temporal changes in the lithospheric thermal regime. Thermal parameters: thermal conductivity; basin = 2.25 Wm-1 K-l; basement = 3 Wm -1 K-l; heat flow; crust = 40 mW m -2, mantle = 20 mW m -2. After Sandiford & Hand (1998) and Hand & Sandiford (1999). c. 10% higher than adjacent stronger regions driven by velocities specified along the plate of lithosphere. Braun & Shaw present the results boundaries during a 200 Ma time period starting of a thin-plate model of the Australian con- in the Ordovician (470 Ma). The results suggest tinental lithosphere in which deformation is that intracratonic deformation occurs at the Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

6 R. E. HOLDSWORTH ET AL. interface between regions of contrasting litho- crucially on datasets from orogenic belts that spheric strength and/or in zones of pre-existing provide constraints on the duration of deforma- weakness caused by previous tectonic activity. tion, its kinematic character and the prevailing Strain localization can arise due to repeated thermal regimes. Hand & Buick and Scrimgeour deformation episodes in a volume of lithosphere & Raith present case studies from central Aus- and, in some cases, may result from the tralia that highlight different stages of the poly- constructive interaction between plate tectonic tectonic evolution of central Australia. Hand & forces that originate on separate plate margins. Buick present a multidisciplinary case study from Sandiford et al. propose a link between the the Reynolds-Anmatjira Range region of the locus of intraplate orogeny in central Australia Arunta Inlier, which has undergone four tecto- and the presence of thick sedimentary succes- nothermal cycles over an interval of c. 1450 Ma. sions deposited in widespread pre-orogenic intra- Up to three Proterozoic events produced a single cratonic basins. This 'tectonic feedback' reflects regional foliation, axial planar to simple large- the long-term thermal and mechanical conse- scale folds. Associated metamorphic assemblages quences of changes in the distribution of heat increase in grade smoothly from northwest to producing elements induced by earlier tectonism. southeast, producing a pattern of isograds that During intraplate orogenesis, deep crustal rocks is remarkably similar to those formed during were exposed by erosion of the heat-producing the later mid-Palaeozoic Alice Springs Orog- upper crust, leading to long-term cooling of the eny. Stratigraphical and geochronological data lithosphere. This causes a progressive strength- show that the apparently simple metamorphic ening of the lithosphere that eventually can be pattern in fact arises due to the superimposed sufficient to terminate intraplate orogenesis in metamorphic effects of four unrelated tecto- central Australia by providing a 'thermal lock'. nothermal cycles. This illustrates that the appar- The presence of cool, anomalously strong litho- ent structural and metamorphic character of a sphere may account for the extraordinary gravity terrain does not always offer a reliable guide to anomalies that are associated with the central the degree of reworking, a feature that may Australian intraplate orogens. in part arise due to the coaxial superimposition The role of crustal heat sources in controlling of regional strains during separate orogenic the long-term mechanical evolution of continen- events (for a good example of this in a differ- tal lithosphere is further explored by McLaren & ent setting, see Tavarnelli & Holdsworth 1999). Sandiford, who focus on the observed 300 Ma Scrimgeour & Raith discuss two terrains in the of episodic Palaeoproterozoic tectonic activity eastern Arunta Inlier with distinct metamorphic that occurred prior to effective Mesoproterozoic histories that are separated by a crustal-scale cratonisation in the Mt Isa domain of northern shear system. They show that the juxtaposi- Australia. This tectonic activity resulted in dra- tion of crustal units, with differing ages along a matic changes in the heat production distribution shear zone system, is broadly coincident with the in the crust. Felsic magmatism transferred heat inferred onset of south-vergent compressional producing elements from lower to mid-upper deformation at the start of the Alice Springs crustal levels, leading to lower crustal cooling, Orogeny. Although the zone of Palaeozoic re- and a highly stratified heat production distribu- working is relatively narrow (<5km wide), it tion. The long-term strengthening caused by the appears to form the northern margin of Palaeo- progressive concentration of heat-producing ele- zoic high-grade intraplate deformation in cen- ments in the upper crust eventually led to effec- tral Australia, and therefore represents a major tive cratonization due to the combined processes tectonic boundary. of erosional stripping of heat production and exhumation of the heat production concentra- tions. At various times prior to cratonization, strengthening was countered by episodes of basin Geochronological and metamorphic formation that led to burial of existing crustal perspectives on continental reworking heat production and increases in total heat production due to the accumulation of sediments. One of the major challenges faced when working The resulting thermal weakening effect may in orogenic belts that have undergone rework- have helped to localize subsequent contractional ing or reactivation is distinguishing between the deformation events expressed by the Mesopro- products of different events. Failure to do this terozoic Isan Orogeny. may result in erroneous deductions regarding the Conceptual studies have resulted in a greater evolution of a terrain, and by inference incorrect understanding of the mechanics of continen- assumptions regarding the processes that con- tal reworking and reactivation, but they rely trolled the evolution. A good example of this Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

CONTINENTAL REACTIVATION AND REWORKING: AN INTRODUCTION 7 comes from the late 1970s to late 1980s when sode (c. 600 Ma) was followed by Pan-African there was a huge increase in the use of meta- high-grade metamorphism, ductile deformation morphic reaction textures to infer tectonother- and later medium-grade retrogression (c. 570 to mal evolutions. In many cases, metamorphic 520 Ma). This was followed by a second Pan- assemblages produced during unrelated meta- African igneous episode (c. 500Ma) during morphic events were linked together, producing which predominantly post-kinematic granitoids apparent tectonic histories that had little rele- were intruded. vance to the real evolution (e.g. Warren 1983; Reworking and reactivation of continental Hensen & Zhao 1995). In some cases, these crust often involves important episodes of fluid apparent tectonic histories led to the postulation flow that are in many cases, of central impor- of causative mechanisms for tectonism that sub- tance in controlling the reactivation of shear sequent work has shown are unlikely to have zone and fault systems. Cartwright et aL dis- applied (e.g. Williams et al. 1995). cuss the effects of fluid flow during the poly- The identification of features such as sim- metamorphic evolution of the Reynolds Range ply metamorphosed cover sequences overlying in the Arunta Inlier in central Australia. During polymetamorphic basement complexes provides contact metamorphism at around 1.78 Ga, igne- compelling evidence for tectonic rejuvenation. ous and surface-derived fluids interacted with In general, however, geochronology is relied the country rocks adjacent to granite plutons. upon heavily to identify the products of indivi- During cooling following regional metamorph- dual tectonic histories. In a review paper, Parrish ism at around 1.59 Ga, fluids were derived pre- illustrates how mineral chronometers, especially dominantly from crystallization of partial melts accessory minerals using the U/Pb isotopic sys- reflecting internal fluid recycling. At around tem, can yield important information regarding 340Ma, during the Alice Springs Orogeny, the environmental conditions and duration of surface-derived fluids infiltrated the middle metamorphic-deformation events during the re- crust. Much of the fluid flow was channelled working of older rocks. A number of examples into shear zones due to increases in intrinsic are presented from a wide variety of geological permeability caused by deformation or reaction environments to illustrate the response of U/Pb enhancement. Johnstone & Harris use oxygen isotope systematics within accessory minerals and carbon stable isotope data to examine fluid to superimposed deformation, metamorphism sources and fluid-rock interaction during early and/or mineral growth. Manhiea et aL discuss Cambrian thermal reworking of a c. 1200 to the metamorphic and structural history of the 900 Ma orogenic belt in western Dronning Maud Archaean to Palaeoproterozoic Kalahari Craton Land, Antarctica. in central western Mozambique and the adjacent Mesoproterozoic rocks of the Mozambique Belt to the east. The new geochronological data presented by these authors suggests that there are Reworking and reactivation: different many similarities in the timing of events with expressions of the same process? those in western Dronning Maud Land, Ant- arctica which was adjacent to the study area An important issue that is not addressed prior to Gondwana breakup. Zhao et aL show specifically by the papers in this volume is the that Archaean mafic granulites from the Palaeo- question of scale in determining whether an proterozoic Trans-North China Orogen and interval of continental rejuvenation should be adjoining Archaean areas preserve textural regarded in terms of reactivation or reworking. evidence for two granulite facies events invol- This problem is well illustrated by the mid- ving contrasting P-T paths, one clockwise, the Palaeozoic intracratonic Alice Springs Orogeny other anticlockwise. These events are correlated in central Australia. From a continental-scale with recognized regional episodes in the adjacent perspective, the Alice Springs Orogen has the Archaean rocks and are used to suggest that appearance of a region of diffuse continental the polymetamorphic granulites were derived deformation with dimensions of at least 1000 km from the reworking of the 2.5Ga metamor- by 250 km. However, at the map scale the vast phosed granulites during the final amalgama- bulk of deformation, and the associated meta- tion of the North China Craton c. 1.8 Ga. Paeeh morphic expression of the tectonic rejuvenation, discusses the geological history of central Dron- is localized within shear zones that can be shown ning Maud Land, Antarctica which consists in important cases to pre-date the Alice Springs of Grenvillian basement rocks intruded by char- Orogeny (e.g. Shaw & Black 1991). Between these nockitic granitoids and anorthosites during two shear zones, Palaeo- to Mesoproterozoic rocks Pan-African igneous episodes. The earlier epi- are largely unaffected by later events. Therefore, Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

Primary Rejuvinated Strain Style of tectonic rheological control fault rock/fabric distribution Strength rejuvenation on rejuvenation

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BASE OF LITHOSPHERE Fig. 2. Schematic diagram depicting the fault rocks/fabrics, strain distribution, tectonic style and primary rheological controls during tectonic rejuvenation at different depths, and how they are related to the distribution of strength in the continental lithosphere. The boundary between the zones of reactivation and reworking is gradational. Its' nature and location may be largely determined by the strength of the lower crust. Note that the strength profile is for 'average' continental lithosphere: large scale tectonic processes, such as orogenesis, delamination and rifting, that perturb the geothermal gradient will lead to significant changes in the strength of the mantle section. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

CONTINENTAL REACTIVATION AND REWORKING: AN INTRODUCTION 9 is the Alice Springs Orogeny an example of con- or narrow viscous shear zones (the reactivation tinental reworking, or is it reactivation involving zone) that broaden downwards into a region of a large number of existing shear zones? In the diffuse viscous flow (the reworking zone) (Fig. 2). former case it would be relevant to ask why Thus, reactivations of discrete shear zones and regional weakening of the lithosphere occurred faults observed in surface exposures preserving in central Australia leading to the localization of shallower crustal deformation regimes should deformation in that crustal volume (e.g. Sandi- pass downwards into a zone of simultaneous ford & Hand 1998; Hand & Sandiford 1999). reworking in the underlying lower crust. Simi- On the other hand, if the Alice Springs Orog- larly, reworking events in lower crustal rocks eny is viewed from a reactivation perspective, now exposed at the surface may have once been strain localization mechanisms may have been expressed as reactivations in the now eroded controlled by shear zone geometry and rheol- shallower parts of the ancient crust. Another ogy of the pre-existing fault rocks (e.g. Korsch example of where reworking and reactivation et al. 1998). may simply be a different expression of the same Orogenic belts typically preserve superimposed process occurs during syn-tectonic magmatism. tectonic, metamorphic and magmatic sequences Melting of existing continental lithosphere is a that formed at different crustal depths during dramatic thermal expression of reworking. How- either burial or exhumation. In this context, ever, the emplacement of magmatic rocks com- reworking and reactivation could be viewed as monly exploits pre-existing structures and may different expressions of the same process (Fig. 2). promote transient fault weakening during em- The classical crustal fault zone model of Sibson placement (e.g. Handy et al. 2001). Thus the (1977), and the results of numerous case studies vastly different processes of magma generation (e.g. Scrimgeour & Close 1999; F16ttmann et al. (reworking) and emplacement (reactivation) are 2001) suggest that deformation in the upper and both essentially expressions of the thermal reju- middle crust is localized along frictional faults venation of existing crust.

100

200

300 depth (km)

I i I 200km -5 Velocity pertubation (%) +5 Fig. 3. Shear wave-speed structure in the vicinity of the Proterozoic Broken Hill Block in eastern Australia (from van der Hilst et al. 1998). The N-S cross-section is located at 143°E and shows large lateral variations in wave speed, and a low velocity zone extending from at least 400 km depth to the near surface separating two high velocity Proterozoic blocks. This low velocity zone may correspond to a region of elevated temperature or highly foliated mantle that is weak relative to the surrounding rocks. The low velocity structure occurs below the surface expression of the long-lived Tasman Line, which is the late Neoproterzoic rifted margin of the Australian continent. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

10 R. E. HOLDSWORTH ET AL.

More fundamentally, it is relevant to consider References the distribution of strength in the continental lithosphere based upon extrapolation of experi- AVOUAC, J. P. & BUROV, E. B. 1996. Erosion as a mental deformation studies (Fig. 2, central col- driving mechanism of intracontinental mountain umn; see Kohlstedt et al. 1995 for a review). These growth. Journal of Geophysical Research, 101, profiles are, at best, approximations of reality, 17 474-17 769. but they do suggest that the majority of the BELL, T. H. & MARES, V. M. 1999. Correlating deformation and metamorphism around orogenic arcs. lithospheric strength resides in the upper man- American Mineralogist, 84, 1727-1740. tle for typical continental lithosphere (Molnar BRAUN, J. & SHAW, R. n. 1998. Contrasting styles of 1992). The existence of large-scale anisotropies in lithospheric deformation along the northern the upper mantle (e.g. Fig. 3; Houseman & Mol- margins of the Amadeus Basin, central Australia. nar 2001) suggests that many examples of con- In'. BRAUN, J., DOOLEY, J., GOLEBY, B., VAN DER tinental reworking may simply be the diffuse HILST, R. & KLOOTWlJK, C. (eds) Structure and expression of movements within long-lived zones Evolution of the Australian Continent. American or volumes of relative weakness located below the Geophysical Union, Geodynamics Series, 26, Moho. The 3-D distribution of strength in the 139-156. BUTLER, R. W. H., HOLDSWORTH, R. E. & LLOYD, lithospheric mantle is profoundly influenced by G. E. 1997. The role of basement reactivation its' thermal structure and history, but it is likely during continental deformation. Journal of the that continental deformation will in general be Geological Society, London, 154, 69-71. driven by the development of broad shear zones DICKSON CUNNINGHAM, W., WINDLEY, B. F., DORJ- in this region. These displacements will be trans- NAMJAA, D., BADAMGAROV, J. & SAANDAR, M. ferred upwards into broad regions of reworking 1996. Late Cenozoic transpression in southwest- in the lower crust and reactivation at still shal- ern Mongolia and the Gobi-Altai-Tien Shan con- lower crustal depths (cf. Teyssier & Tikoff 1998, nection. Earth and Planetary Science Letters, 140, figs 8 & 9). If pre-existing weaknesses are present, 67-81. ENGLAND, P. C. 1987. Diffuse continental deforma- particularly in the secondary load-bearing region tion; length scales, rates and metamorphic evolu- of the mid-crust (Fig. 2), these will preferentially tion. Philosophical Transactions of the Royal accommodate displacements, but their presence Society of London, 321, 2-22. may not be essential to the process of tectonic ENGLAND, P. C. & HOUSEMAN, G. A. 1985. The role of rejuvenation. lithospheric strength heterogeneities in the tec- In summary, the structural, metamorphic and tonics of Tibet and neighbouring regions. Nature, compositional character of the continents is 315, 297-301. shaped by the way in which continental litho- ENGLAND, P. C. & JACKSON, J. 1989. Active deforma- sphere undergoes tectonic rejuvenation. Rejuve- tion of the continents. Annual Reviews of Earth and Planetary Sciences, 17, 197-226. nation is achieved by two end-member processes: FLOTTMANN, T., HAND, M., CLOSE, D., EDGOOSE, C. reactivation and reworking. In reality, these may & SCmMGEOUR, I. 2001. Thrust tectonic styles of simply be different manifestations of the same the intracratonic Alice Springs and Petermann process with the appropriateness of using these orogenies, central Australia. In: MCCLAY, K. R. two terms determined by: (a) the observational (ed.) Thrust Tectonics and Petroleum Systems. scale applied to a specific problem; and/or (b) the American Association of Petroleum Geologists, depth at which the exposed deformation system Tulsa. formed in the lithosphere. As a result, reworking HAND, M. & SANDIFORO, M. 1999. Intraplate defor- and reactivation convey important information mation in central Australia, the link between subsidence and fault reactivation. Tectonophysics, regarding the styles and controls of tectonic 305, 121-140. rejuvenation, and are therefore fundamental in HANDY, M. R, MULCH, R., ROSENAU, M. & ROSEN- improving our understanding of continental tec- BERG, C. R. 2001. The role of fault zones and tonic processes. In many ways, these processes melts as agents of weakening, hardening and exemplify the fundamental differences that exist differentiation of the continental crust a syn- between plate tectonics, which is largely based thesis. In: HOLDSWORTH, R. E., STRACHAN, R. A., on the dynamics of oceanic lithosphere, and the MAGLOUGHLIN, J. F. & KNIPE, R. J. (eds) The large-scale behaviour of continental lithosphere Nature and Tectonic Significance of Fault Zone (Molnar 1988). Weakening. Geological Society, London, Special Publications, 186, 303-330. We would like to thank all the contributors to the HENDRIX, M. S., GRAHAM, S. A., CARROLL, A. R., present volume and those who organized and attended SOBEL, E. R., MCKNIGHT, C. L., SCHULEIN, B. J. the Alice Springs conference and associated field & WANG, Z. 1992. Sedimentary record and excursions. Ken McCaffrey and Andy Morton are climatic implications of recurrent deformation in thanked for their comments on earlier versions of this the Tian Shan; evidence from Mesozoic strata of manuscript. the north Tarim, south Junggar, and Turpan Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021

CONTINENTAL REACTIVATION AND REWORKING: AN INTRODUCTION 11

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